Machines that are operative in all four quadrants of the plane of speed of rotation-moment of rotation and that are switched off from the external power supply need to have short circuit protection, in the event a short circuit occurs in the d.c. current power supply. When fuses are used that blow as a result of a short circuit, they must be replaced, which is time-consuming. If instead of fuses, mechanical circuit components are used that have significant response delays, the short-circuit current can attain unacceptably high levels by the time the energy supply is interrupted.
A problem is to provide a safe short-circuit protection device with quick acting means that do not need to be replaced for electrical machines fed by a pulse-controlled a.c. converter. It is to be understood that such a pulse-controlled a.c. converter usually contains an intermediate circuit capacitor and recovery diodes, as well as individually controllable semiconductors.